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Crystal is composed of molecules, ions and atoms which are properly ordered to form a transparent internal structure. There are three most common types of crystals which are often used for experiments or you can do grow the at home; Alum Crystals, Salt Crystals and Sugar Crystals.

Growing Alum Crystals
Step 1
Buy yourself alum; it should be classified under spices in the grocery stores.
Step 2
Pour ½ to ¾ cup of boiling water into the jar. Be sure to inspect the jar for solid particles or dirt.
Step 3
Dissolve 2 ½ tbsp of alum. Make sure that it is properly dissolved.
Step 4
Let the jar with alum solution untouched overnight or about 8 hours.
Step 5
Transfer the liquid solution into another clean jar.
Step 6
In the first jar, you will see crystals at the bottom of the jar. You may select a larger one or the best that you could see.
Step 7
Tie the one you have chosen with nylon; the one used for fishing because it also has a clear strand.
Step 8
Hang the piece of crystal and submerge it into the alum solution you transferred to another jar. Be sure not to let it touch the sides nor the bottom of the jar.
Step 9
Let it grow until you are fully satisfied with its size then remove it from the solution and hang to dry.

For other types of crystals see the following guides:

How to Grow Sugar Crystals